THIN FILMS MADE FROM COLLOIDAL ANTIMONY TIN OXIDE NANOPARTICLES FOR TRANSPARENT CONDUCTIVE APPLICATIONS

A Thesis Presented toThe Academic Faculty

by

Abigail Halim

In Partial Fulfillmentof the Requirements for the Degree

Bachelor of Science in theSchool of Materials Science and Engineering

Georgia Institute of TechnologyMay 2013

THIN FILMS MADE FROM COLLOIDAL ANTIMONY TIN OXIDE NANOPARTICLES FOR TRANSPARENT CONDUCTIVE APPLICATIONS

Approved by:

Dr. Rosario Gerhardt, Advisor School of Materials Science and Engineering Georgia Institute of Technology

Dr. Dong Qin _.,. School of Materials Science and Engineering

Georgia Institute of Technology

Dr. Fred Cook School of Materials Science and Engineering Georgia Institute of Technology

Date Approved: April 26, 2013

ACKNOWLEDGMENTS

! I would like to thank Dr. Gerhardt for her guidance and support and Danny

Jeong, Salil Joshi, Tim Pruyn, Rachel Muhlbauer, Chunqing Peng, and Justin Brandt for

training me on various instruments and answering my questions. I would also like to

thank Dr. Kroger for allowing me to use his Zetasizer and Nick Haase for training and

helping me in using the Zetasizer. Lastly, I would like to thank Exide Technologies for

funding this project through a research scholarship and the Institute Presidentʼs Office

for granting a salary and travel award through the Presidentʼs Undergraduate Research

Award.

iii

TABLE OF CONTENTS

! ! ! ! ! ! ! ! ! ! ! ! Page

ACKNOWLEDGMENTS ! ! ! ! ! ! ! ! ! iii

LIST OF TABLES! ! ! ! ! ! ! ! ! ! v

LIST OF FIGURES ! ! ! ! ! ! ! ! ! ! vi

LIST OF SYMBOLS AND ABBREVIATIONS ! ! ! ! ! ! vii

SUMMARY! ! ! ! ! ! ! ! ! ! ! viii

CHAPTER

! 1 INTRODUCTION! ! ! ! ! ! ! ! ! 1

! 2 LITERATURE REVIEW ! ! ! ! ! ! ! ! 2

! 3 MATERIALS AND METHODS! ! ! ! ! ! ! 6

! 4 RESULTS AND DISCUSSION! ! ! ! ! ! ! 8

! 5 CONCLUSIONS! ! ! ! ! ! ! ! ! 14

REFERENCES! ! ! ! ! ! ! ! ! ! 15

iv

LIST OF TABLES

! ! ! ! ! ! ! ! ! ! ! ! Page

Table 2.1 Common TCO Materials! ! ! ! ! ! ! 2

Table 2.2 Summary of ATO Thin Film Properties! ! ! ! ! 5

v

LIST OF FIGURES

! ! ! ! ! ! ! ! ! ! ! ! Page

Figure 3.1! ! ! ! ! ! ! ! ! ! ! 7

Figure 4.1! ! ! ! ! ! ! ! ! ! ! 8

Figure 4.2! ! ! ! ! ! ! ! ! ! ! 9

Figure 4.3! ! ! ! ! ! ! ! ! ! ! 9

Figure 4.4! ! ! ! ! ! ! ! ! ! ! 10

Figure 4.5! ! ! ! ! ! ! ! ! ! ! 11

vi

LIST OF SYMBOLS AND ABBREVIATIONS

ATO! ! ! ! ! ! ! ! antimony tin oxide

TMAH!! ! ! ! ! ! ! tetramethylammonium hydroxide

ITO! ! ! ! ! ! ! ! indium tin oxide

TCO! ! ! ! ! ! ! ! transparent conducting oxide

vii

SUMMARY

! In this study, antimony tin oxide (ATO) nanoparticles from Alfa Aesar were

redispersed in water using tetramethylammonium hydroxide (TMAH) as a dispersing

agent and deposited onto glass substrates by spin coating. Films of one to five layers

were made. These thin films were characterized using impedance spectroscopy and

ultraviolet-visible spectroscopy to obtain their resistances and optical transmittance,

respectively. The films displayed sheet resistances around 105-104 kΩ/� and optical

transmittance in the near infrared to near ultraviolet range above 95%. Films were then

made using a higher concentration ATO solution and found to achieve sheet resistances

on the order of 102 kΩ/�. Impedance measurements, along with optical micrographs,

were taken at different locations on the films, demonstrating that films of more than one

layer showed greater uniformity. Further sets of films were also produced with varying

substrate preparation and dispersion deposition parameters. Aside from dispersion

concentration, high humidity during film measurement was found to be the most crucial

parameter for achieving low sheet resistances.

viii

CHAPTER 1

INTRODUCTION

! Oxide thin films have gained much attention for transparent conductive

applications due to their unique electrical and optical properties. Indium tin oxide (ITO)

thin films have become an industry standard but require scarce, expensive materials to

produce. Antimony-doped tin oxide thin films are also being studied as an alternative, as

they have properties comparable to ITO thin films and are less expensive.

1

CHAPTER 2

LITERATURE REVIEW

Transparent Conducting Oxides

! Transparent conducting oxides (TCOs) are thin films of metal oxides that exhibit

both optical transparency and electrical conductivity [1]. For these reasons, they have

become widely used in applications such as solar cells, light emitting diodes, and liquid

crystal displays [2]. The first TCO was discovered over a century ago when a sputter

deposited cadmium thin film underwent incomplete thermal oxidation with post-

deposition heating in air [3]. Since this realization, many other TCO materials have been

developed, namely n-type semiconductors with tin oxide, indium oxide, or zinc oxide as

the main component, displayed in Table 2.1 [1].

Table 2.1 Common TCO Materials

Main Component Dopant

Tin Oxide (SnO2)Fluorine (F)

Tin Oxide (SnO2)Antimony (Sb)

Indium Oxide (In2O3)Tin (Sn)

Indium Oxide (In2O3)Zinc (Zn)

Zinc Oxide (ZnO)

Aluminum (Al)

Zinc Oxide (ZnO) Boron (B)Zinc Oxide (ZnO)

Gallium (Ga)

2

! The key to attaining both optical transparency and electrical conductivity in TCOs

is the nature of the energy band gap in degenerate semiconductors. To overcome the

band gap, electrical conductivity in TCOs is achieved by increasing the number of free

charge carriers through intrinsic defects, such as oxygen vacancies, or through extrinsic

dopants, typically higher-valency metal cations [2]. In n-type semiconductors, these

dopants or defects provide energy levels close to the bottom of the conduction band,

allowing those electrons to get promoted into the conduction band as free charge

carriers. In p-type semiconductors, the additional energy levels lie close to the valence

band, allowing electrons to be promoted into the extra energy levels and creating holes

in the valence band to act as free charge carriers [3].

! The optical transparency of TCOs is made possible by the existence and

magnitude of the energy band gap. Unlike metals, there exists a range of wavelengths

for which electrons cannot absorb light due to their corresponding energies and the

semiconductor band gap energy [4]. This creates a transmission boundary in the near-

ultraviolet region, above which light has enough energy to promote an electron in the

valence band into the conduction band [2]. The lower energy boundary appears in the

near-infrared region, below which transmission is restricted due to absorption of light by

electronic transitions between energy levels within the valence band [3].

! Indium tin oxide (ITO) is an In2O3-rich compound of In2O3 and SnO2 that has

become the principal TCO material as a result of its excellent optical and electrical

properties, having average resistivities of about 1 × 10−4 Ω⋅cm [2]. However,

alternatives to ITO are being investigated because of the high cost and scarcity of

indium. Fluorine-doped tin oxide (FTO) is the second most used TCO, but it has a

3

relatively low electrical conductivity and is more difficult to pattern using wet-etching [2].

Aluminum- and gallium-doped zinc oxides (AZO and GZO, respectively) are also good

alternatives to ITO, but they tend to degrade faster than ITO and FTO in hot, moist

atmospheres [2].

Antimony Tin Oxide TCOs

! Antimony tin oxide (ATO) is a TCO material whose properties have been studied

and looks to be a promising alternative to ITO due to its low cost [5-12]. Where ITO is

Sn-doped indium oxide, ATO is Sb-doped tin oxide (SnO2). SnO2 has a rutile crystal

structure, with Sn atoms in the corner and center positions and oxygen atoms

occupying the tetrahedral interstitial sites [13]. In ATO, Sb cations substitute for the Sn4+

cations. With low amounts of Sb doping, Sb5+ ions dominate, decreasing resistivity by

increasing the free carrier density with excess electrons. With increased doping (above

5 wt.% Sb), Sb5+ is reduced to Sb3+, and Sb3+ ions begin to dominate, increasing

resistivity [14].

! Table 2.2 displays a summary of the optimal electrical and optical properties

obtained from ATO thin films of different compositions using a variety of deposition

techniques.

4

Table 2.2 Summary of ATO Thin Film Properties

Deposition Technique Composition Resistivity/Sheet

Resistance Transmission Reference

Sol-gel spinning 30 at.% Sb 1.19 × 10-3 Ω·cm 92.5% at 0.55 μm 5

Spin coating 5 mol% Sb 1.7 × 10-2 Ω·cm >90% in visible range 6

Spray pyrolysis 0.5 mol% Sb 2.57 × 10-3 Ω·cm ~90% in visible and near-IR range 7

Solvothermal 15 at.% Sb 9.0 × 105 Ω/ - 8

Reactive DC magnetron sputtering

5 wt.% Sb 3.3 × 10-3 Ω·cm 74% in visible range 9

Perfume atomizer 2 at.% Sb40 at.% F 3.27 × 10-4 Ω·cm 76% in visible

range 10

Oblique angle deposition 7 wt.% Sb 9.0 × 102 Ω/ ~90% in visible

range 11

Sol-gel dip-coating 15 mol% Sb 1.0 × 102 Ω/ - 12

While the resistivities and transmission for ATO thin films that have been recorded are

close to those of their ITO counterparts, they still fall behind the lower resistivities and

higher transmission of ITO. To be able to compete with ITO, ATO thin films also need to

be more easily manufactured. This study proposes to promote the use of ATO by

optimizing the fabrication of ATO thin films by solution-processing commercially-

available ATO nanoparticles.

5

CHAPTER 3

MATERIALS AND METHODS

Dispersion Preparation

! ATO (Alfa-Aesar) nanoparticles were redispersed in water using

tetramethylammonium hydroxide (TMAH) as a dispersing agent. The ATO powder was

massed and combined with 10 wt.% TMAH solution (Sigma-Aldrich) in a ratio of 0.1823

μL of TMAH solution per mg of ATO [6]. Water was added to adjust the ATO dispersion

concentration, and the dispersion was sonicated for 10 minutes and stirred until

deposition. Dispersions of 0.1 wt.% and 5 wt.% were made.

Film Deposition

! Microscope slides were cut into 1” x 1” substrates and cleaned using Kimwipes

and pure water, acetone, and isopropyl alcohol. The substrates were treated with UV

ozone at 50°C for 15 minutes. Spin coating was used to deposit the ATO dispersion

onto the substrates. The spin coating program used was an initial speed of 2000 rpm

accelerated at 100 r/s for 45 s., followed by a drying speed of 5000 rpm accelerated at

1000 r/s for 30 s. To form one layer of ATO film using spin coating, 200 μL of the ATO

suspension was pipetted onto the center of the substrate before starting the spin

coating program. To ensure all water had been removed before deposition of further

layers, the films were placed in a furnace set to 90-100°C for 10 minutes between each

layer deposition. Films of one to five layers were made.

6

Varying Substrate Preparation and Film Deposition Parameters

The importance of UV-ozone treatment for substrate preparation was studied by

exposing some substrates to UV ozone at 50°C for 15 minutes and leaving other

substrates untreated. The deposition conditions studied were pipetting the dispersion

onto the substrate before starting the spin coating program, while the substrate was

motionless, and pipetting the dispersion 3 s. into the spin coating program, while the

substrate had a velocity of about 300 rpm.

Film Characterization

! The thin films were characterized using impedance spectroscopy to obtain their

sheet resistances. A Solartron 1296 and Solartron 1260 were used to take four point

probe measurements on the samples. Measurements were taken between probes 1-2,

2-3, and 3-4 in at least one of the positions shown in Figure 3.1. The sheet resistances

were calculated by fitting the first semicircle in the complex impedance graph from the

impedance data to a simple equivalent circuit of a resistor in parallel with a constant

phase element. All measurements from one film were averaged to obtain the

representative sheet resistance of each film. Optical microscopy was used to investigate

the film structures and uniformity.

Figure 3.1. Schematic of the five positions used on the films to place the four point

probe for impedance measurements.

1 45

3

2

7

CHAPTER 4

RESULTS AND DISCUSSION

! Sheet resistances of films made with 0.1 wt.% and 5 wt.% ATO dispersions are

displayed in Figure 4.1a. Examples of the impedance curves for the 5 wt.% films from

which the sheet resistances were calculated are shown in Figure 4.1b.

Figure 4.1 a) Average sheet resistances of ATO thin films of one to five layers, with

diamonds showing 0.1 wt.% films and circles showing 5 wt.% films and b)

representative impedance graphs of films made with 5 wt.% ATO dispersion. Inset

shows a larger view of the semicircles for films of 2-5 layers.

All 0.1 wt.% films showed high sheet resistances on the order of 106 kΩ/�, indicating

that the ATO nanoparticles did not form a continuous network in the film. It can clearly

be seen that increasing the dispersion concentration drastically affects the sheet

resistance, decreasing it by about four orders of magnitude. With the 5 wt.% films, the

1E+05

1E+06

1E+07

1E+08

1E+09

1E+10

0 1 2 3 4 5 6Aver

age

Shee

t Res

istan

ce (kΩ

/�)

# of Layers

1x107

1x106

1x105

1x104

1x103

1x102 0E+00

5E+06

1E+07

2E+07

2E+07

0E+005.0E+061.0E+071.5E+072.0E+07

-Z” (

kΩ)

Zʼ (kΩ)0 5.0x103 1.0x104 1.5x104 2.0x104

2.0x104

1.5x104

1.0x104

5.0x103

0

(a)

(b)

8

sheet resistances level out after two layers. This may be due to repulsion of the film and

the dispersion, preventing the actual addition of further layers.

! Optical micrographs of the 5 wt.% films are displayed in Figure 4.2.

Figure 4.2 Optical micrographs of films made with 5 wt.% ATO dispersion. 10μm scale

bar applies to all images.

The micrographs show that all films are fairly uniform and exhibit the same structure.

The visible formations are likely aggregates of the ATO nanoparticles. Single

nanoparticles are not visible on this scale and could be spread throughout the regions

between the aggregates.

! Figure 4.3 presents the transmittance of the 0.1 wt.% and 5 wt.% films.

Figure 4.3 Transmittance spectra of ATO films of 1-5 layers made with a) 0.1 wt.% and

b) 5 wt.% ATO dispersions.

1 Layer 2 Layer 3 Layer 4 Layer 5 Layer

90

95

100

105

110

250 420 590 760 930 1100

% T

rans

mitt

ance

Wavelength (nm)

60

70

80

90

100

250 420 590 760 930 1100

% T

rans

mitt

ance

Wavelength (nm)

(a) (b)

9

A substantial decrease in transmittance can be seen when the dispersion concentration

is increased from 0.1 wt.% to 5 wt.%. This decrease is understandable, as more

nanoparticles are deposited onto the substrate when using a higher concentration.

Effects of Substrate Preparation and Deposition Parameters

! When studying the effects of UV-ozone treatment and of deposition before or

after the start of the spin coating program, minor differences in properties were

detected. The sheet resistances and photographs of films with different preparation

parameters are shown in Figure 4.4.

Figure 4.4 a) Average sheet resistances and b) photographs of films with varying

substrate preparation and dispersion deposition parameters.

1E+05

4E+05

6E+05

9E+05

1E+06

Aver

age

Shee

t Res

istan

ce (kΩ

/�) 1x103

9x102

6x102

4x102

1x102 UV pipet

before

No UV pipet

before

UV pipet after

No UV pipet after

UV - pipet before

UV - pipet after

No UV - pipet after

No UV - pipet before

(a)

(b)

10

The lowest sheet resistance was obtained with a substrate prepared with UV-ozone and

deposition after starting the spin coating program. Treating the glass with UV-ozone

activates the surface, making it more hydrophilic and enabling the aqueous dispersion

of ATO nanoparticles to better adhere to the glass. The films on UV-ozone treated

substrates also looked more uniform and completely covered the substrate surface,

whereas the other films left some of the glass surface uncovered.

Effect of Humidity

! When attempting to reproduce the films with sheet resistances on the order of

102 kΩ/�, some experiments resulted in films of higher sheet resistances by five orders

of magnitude. All substrate preparation, dispersion preparation, and deposition

parameters were kept constant, so other factors were investigated. It was found that

relative humidity during measurement significantly affected the sheet resistances of the

films. The sheet resistances and representative impedance graphs of the same films

measured at low and high relative humidities are displayed in Figure 4.5.

11

Figure 4.5 a) Sheet resistances of films measured at low (diamonds) and high (circles)

relative humidities. Impedance plots of 5 layer films measured with b) low and c) high

relative humidities.

It is evident that films measured at a higher relative humidity had lower sheet

resistances. It is unclear why this change in properties occurs, but it is likely related to

the residual TMAH that is contained in the films.

1E+05

1E+06

1E+07

1E+08

1E+09

1E+10

1E+11

0 1 2 3 4 5 6

Shee

t Res

istan

ce (kΩ

/�)

# of Layers

(a)1x108

1x107

1x106

1x105

1x104

1x103

1x102 0E+00

6E+05

1E+06

0E+006.0E+051.2E+061.8E+062.4E+06

-Z” (

kΩ)

Zʼ (kΩ)

0E+00

5E+09

9E+09

0E+004.5E+099.0E+091.4E+101.8E+10

-Z” (

kΩ)

Zʼ (kΩ)

(b)

(c)

0 4.5x106 9.0x106 1.4x107 1.8x107

9.0x106

4.5x106

0

0 6.0x102 1.2x103 1.8x103 2.4x103

1.2x102

6.0x102

0

12

CHAPTER 5

CONCLUSIONS

! Films of sheet resistances on the order of 102 kΩ/� were produced using

solution processed commercial ATO nanoparticles. Increasing the ATO dispersion

concentration decreased the sheet resistances of the films. Although deliberately

varying substrate preparation and deposition parameters had some effect on film

properties, the most important factor in producing low or high sheet resistance films was

relative humidity during film measurement.

! Although the ATO films produced in this study did not show comparable

properties to those of ITO films, the decrease in sheet resistances achieved by varying

processing parameters shows promise in further reducing the ATO sheet resistances to

values necessary for current applications. For application environments like solar cells,

it would also be necessary to eliminate the effect of relative humidity on the ATO film

properties, as relative humidity would constantly vary in open air. This would likely

require the removal of residual TMAH from the film while maintaining film structure and

uniformity. With further studies to resolve these issues, commercial ATO nanoparticles

could potentially be solution-processed into transparent conductive films.

13

REFERENCES

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[2]! H. Liu, V. Avrutin, N. Izyumskaya, U. Ozgur, and H. Morkoc (2010). "Review: Transparent conducting oxides for electrode applications in light emitting and absorbing devices." Superlattices and Microstructures 48: 458-484.

[3]! G. J. Exarhos and X. Zhou (2007). "Review: Discovery-based design of transparent conducting oxide films." Thin Solid Films 515: 7025-7052.

[4]! Schaffer, Saxena, Antolovich, Sanders, Warner (2010). The Science and Design of Engineering Materials, McGraw-Hill.

[5]! L. K. Dua, A. De, S. Chakraborty, and P. K. Biswas (2008). "Study of spin coated high antimony content Sn-Sb oxide films on silica glass." Materials Characterization 59(5): 578-586.

[6]! C. Goebbert, R. Nonninger, M. A. Aegerter, and H. Schmidt (1999). "Wet chemical deposition of ATO and ITO coatings using crystalline nanoparticles redispersable in solutions." Thin Solid Films 351(1-2): 79-84.

[7]! G. Jain and R. Kumar (2004). "Electrical and optical properties of tin oxide and antimony doped tin oxide films." Optical Materials 26(1): 27-31.

[8]! H. J. Jeon, M. K. Jeon, M. Kang, S. G. Lee, Y. L. Lee, Y. K. Hong, and B. H. Choi (2005). "Synthesis and characterization of antimony-doped tin oxide (ATO) with nanometer-sized particles and their conductivities." Materials Letters 59(14-15): 1801-1810.

[9]! J. Montero, C. Guillen, and J. Herrero (2011). "Discharge power dependence of structural, optical and electrical properties of DC sputtered antimony doped tin oxide (ATO) films." Solar Energy Materials and Solar Cells 95(8): 2113-2119.

[10]! K. Ravichandran and P. Philominathan (2008). "Fabrication of antimony doped tin oxide (ATO) films by an inexpensive, simplified spray technique using perfume atomizer." Materials Letters 62(17-18): 2980-2983.

[11]! X. Xiao, G. Dong, J. Shao, H. He, and Z. Fan (2010). "Optical and electrical properties of SnO2:Sb thin films deposited by oblique angle deposition." Applied Surface Science 256(6): 1636-1640.

14

[12]! D. Zhang, Z. Deng, J. Zhang, and L. Chen (2006). "Microstructure and electrical properties of antimony-doped tin oxide thin film deposited by sol-gel process." Materials Chemistry and Physics 98(2-3): 353-357.

[13]! G. Qin, D. Li, Z. Chen, Y. Hou, Z. Feng, and S. Liu (2009). "Structural, electronic and optical properties of Sn1-xSbxO2." Computational Materials Science 46(2): 418-424.

[14]! T. Krishnakumar, R. Jayaprakash, N. Pinna, A. R. Phani, M. Passacantando, and S. Santucci (2009). "Structural, optical and electrical characterization of antimony-substituted tin oxide nanoparticles." Journal of Physics and Chemistry of Solids 70: 993-999.

15